The present invention relates to optical tracking systems and methods based on passive markers. In particular the present invention relates to improvements in passive markers themselves and to improvements in accuracy of passive marker based methods and systems.
Prior art optical systems utilize data captured from image sensors to determine the 3D position of an object imaged by one or more cameras calibrated to provide overlapping projections. Data acquisition is traditionally implemented using special markers attached to the object or adhering to the object. These systems produce data with 3 degrees of freedom for each marker, and rotational information must typically be inferred from the relative orientation of three or more markers. Some prior art systems are disclosed, for instance, by US 2011/0254922 A1 or WO 2004/002352 A2.
Known passive optical systems use markers coated with a retroreflective material to reflect light that is generated near the camera lens. The threshold of the camera can be adjusted so that only the bright reflective areas will be sampled.
Typically, the known markers have a checkered bright-and-dark pattern. The centroid of the marker is typically defined by an intersection of straight edges formed between alternating bright and dark regions. The centroid is estimated as a position of the marker within the two-dimensional image that is captured.
An object with markers attached at known positions is used to calibrate the cameras and obtain their positions and the lens distortion of each camera is measured. If two calibrated cameras see a marker, a three-dimensional fix can be obtained (source: Wikipedia).
There are however optical tracking applications requiring a high degree of accuracy, such as medial applications, high-precision mechanical applications, precise constructional components measurements. For example replacement surgery success heavily depends on proper positioning of replacement implants.
Computer assisted surgery (CAS—source: Wikipedia) represents a surgical concept and set of methods, that use computer technology for presurgical planning, and for guiding or performing surgical interventions. CAS is also known as computer aided surgery, computer assisted intervention, image guided surgery and surgical navigation, but these terms are more or less synonyms with CAS. CAS has been a leading factor for the development of robotic surgery. In CAS, computerized models are used to ensure correct objects (such as bones) alignment. Tracking of surgical instruments and components in relation to patient anatomy is also of great importance.
It would be advantageous to improve accuracy of measurement while applying cost-effective passive markers.
U.S. Pat. No. 6,877,239 B2, entitled “Method and device for checking a marking element for displacement” (“239 patent”) discloses a method for checking a marking element for displacement in relation to a holding structure, in particular a bone, on which this marking element is fixed, said marking element being used for determining position in navigational surgery, and said marking element functioning with the greatest possible precision, it is proposed to choose a point of orientation which is in a unique relationship to the holding structure and to monitor the position of the point of orientation in a reference system of the marking element.
According to '239 patent, the position of a marking element in space can be determined via a navigation station. For example, a marking element for this purpose comprises a large number of transmitters such as ultrasound transmitters or infrared transmitters whose signals can be registered by receivers. In particular, three receivers are provided for the three spatial coordinates.
A drawback of the '239 patent is that active markers are used such as ultrasound transmitters or infrared transmitters. This makes the system more expensive and the marker devices require more space in the operating area than passive markers.
Another publication of U.S. Pat. No. 7,780,681 B2, entitled “Non-imaging, computer assisted navigation system for hip replacement surgery” (“681 patent”), discloses a locating system; a computer, interfaced to the locating system and interpreting the positions of tracked objects in a generic computer model of a patient's hip geometry; a software module, executable on the computer, which defines the patient's pelvic plane without reference to previously obtained radiological data, by locating at least three pelvic landmarks; and a pelvic tracking marker, fixable to the pelvic bone and trackable by the locating system, to track in real time the orientation of the defined pelvic plane. Preferably, the system also includes a femoral tracking marker, securely attachable to a femur of the patient by a non-penetrating ligature and trackable by the locating system to detect changes in leg length and femoral offset.
U.S. Pat. No. 6,351,659 B1, entitled “Neuro-navigation system” (“659 patent”) discloses a Neuro-navigation system comprising a reflector referencing system including passive reflectors and a marker system with markers or landmarks wherein the reflectors as well as the markers as regards their shape, size and material selection as well as their arrangement or attachment on the parts of the body to be operatively treated and on the surgical instruments are configured so that mapping their locations is substantially facilitated or is able to take place more accurately positioned by a computer/camera unit having a graphic display terminal as well as the operative treatment with the aid of this unit.
The '659 patent publication also discloses a referencing system for surgical instruments and operative treatment apparatus, said system comprising: a source of infrared radiation; plural cameras for detecting reflected infrared radiation; a computer connected to the cameras; plural adaptors each configured for removable attachment to a respective surgical instrument or operative treatment apparatus, and plural sets of at least three infrared reflectors secured to said plural adaptors. The plural sets of at least three infrared radiation reflectors include at least one spherical reflector.
The '681 and '659 patent publications disclose application of suitable optical markers utilizing multiple reflective spheres. Such passive reflective markers reflect infrared light emitted by suitable infrared illuminators.
Drawbacks of currently used reflective spheres include a need for replacement after single use or even during lengthy surgeries. Further, the reflective spheres are difficult to clean intra-operatively.
Afore-mentioned US 2011/0254922 A1 and DE 10 2006 060 716 A1 each disclose optical markers having an inner marker area providing intersecting edges, and having an outer marker area surrounding the inner marker area. The outer marker area may have an individual bright-and-dark pattern that serves as an identifier. While these passive marker systems avoid the disadvantages of active marker systems, it would be desirable to improve these known systems in order to obtain higher accuracy in a cost-efficient manner.